1. Field of the Invention
The present invention relates generally to a broadband wireless communication system adopting a relay scheme, and in particular, to a frame configuration method and apparatus for providing a relay service in the broadband wireless communication system adopting the relay scheme.
2. Description of the Related Art
Along with the diversification of service types available through mobile terminals, broadband systems have attracted interest and new systems are under development. For deployment of a new system, installation of additional Base Stations (BSs) requires configuration of a new wired network, increasing cost. Therefore, relay communication technology which adds a Relay Station (RS) to relay between a BS and a Mobile Station (MS) is considered as promising.
FIG. 1 illustrates the configuration of a typical cellular system using RSs.
Referring to FIG. 1, an MS 110 within the cell area 101 of a BS 100 communicates directly with the BS 100. On the other hand, an MS 120, which is located outside the cell area 101 of the BS 100 and thus in a poor channel state, communicates with the BS 100 via an RS 130.
The RS 130 provides better-quality radio channels to MSs 110 and 120 when they communicate with BS 100 but in a bad channel state, as they are located at a boundary of the cell area 101. Thus, BS 100 can provide a high-speed data channel to the cell boundary area using a multi-hop relay scheme and thus expand its cell coverage.
Hence, RS 130 serves the purpose of expanding cell area 101 of BS 100 or increasing data rate by diversity effect. RS 130 may operate in an Amplify and Forward (AF) manner or a Decode and Forward (DF) manner.
For communications with RS 130, a frame has to be so configured as to support both a direct link and a relay link.
FIG. 2 illustrates a communication procedure using an RS in a typical Time Division Duplexing (TDD) system.
Referring to FIG. 2, a BS 201 sends data to an MS 205 outside its cell area via an RS 203. BS 201 sends data to RS 203 in step 211 and then RS 203 forwards the data to MS 205 in step 213.
When MS 205 sends uplink data to RS 203 in step 215, RS 203 forwards the received data to BS 201 in step 217.
FIG. 3 illustrates a frame structure for communications via an RS in the typical TDD system. The horizontal axis represents time and the vertical axis represents frequency. An MS connected to a BS via a direct link is called an MSBS, and an MS connected to a BS via an RS is called an MSRS.
Referring to FIG. 3, a frame is divided into downlink and uplink frames.
The downlink frame is broken up into a period 301 for downlink transmission from a BS to an RS or an MSBS, and a period 303 for downlink transmission from the RS to an MSRS. During the period 303, the RS forwards data received from the BS to the MSRS.
The uplink frame is broken up into a period 305 for uplink transmission from the MSRS to the RS and a period 307 for uplink transmission from the RS or the MSBS to the BS.
A guard region is defined between data transmission periods, taking into account delay spread and downlink/uplink switching delay.
As described above, a drawback with the above frame configuration is increased delay with the number of hops between radio ends. When the BS sends data to the MS via the RS, delay between the radio ends is at least doubled. Moreover, the existence of a time guard region between data transmission periods increases overhead.